(a) Field of the Invention
The present invention relates to a method and an apparatus for transmitting a reference signal, a method and an apparatus for measuring and reporting channel state information, and a method of configuring the same.
(b) Description of the Related Art
A full-dimensional (FD) multiple input multiple output (MIMO) transmission adaptively forms a beam for the horizontal domain/direction or azimuth domain/direction (hereinafter collectively referred to as ‘horizontal domain’) and the vertical domain/direction, zenith domain/direction, or elevation domain/direction (hereinafter collectively referred to as ‘vertical domain’) by using a 2D antenna array, thereby enabling an improvement of spectral efficiency, a reduction in inter-cell interference, an improvement of a cell coverage, etc.
For the FD-MIMO transmission, like the current MIMO transmission that adaptively forms the beam only for the horizontal domain, a base station or a relay station must receive a report (or feedback, hereinafter collectively referred to as ‘report’) of channel state information (CSI) including a rank indicator (RI), a channel quality indicator (CQI), a precoding matrix indicator (PMI), positions of M selected subbands (or preferred subbands, hereinafter collectively referred to as ‘selected subbands’), etc. from a terminal.
Meanwhile, the terminal must acquire a MIMO channel for measurement of the CSI. If the base station configures a reference signal (CSI-RS) or a non-zero power (NZP) CSI-RS, and accordingly transmits the CSI-RS, the terminal receives the CSI-RS and estimates the MIMO channel. The base station may configure energy per resource element (EPRE) of the CSI-RS in consideration of performance of channel estimation using the CSI-RS, inter-cell interference due to the transmission of the CSI-RS, etc. If the terminal measures the above-described various types of CSI (for example, the RI, the CQI, the PMI, the selected subbands, etc.) by using the channel estimated from the CSI-RS and reports the measured CSI to the base station, the base station performs scheduling (or resource allocation) including link adaptation by using the CSI. The CSI-RS transmission described below may mean that the base station transmits the CSI-RS to the terminal, and includes the terminal receiving the CSI-RS from the base station. The CSI report described below may mean that the terminal transmits the measured CSI to the base station, and includes the base station receiving the CSI from the terminal. The ‘current standard’ indicated below means the current standard of the 3rd generation partnership project (3GPP) long term evolution (LTE) Release 12.
The CSI-RS is a downlink reference signal transmitted by the base station for the purpose of measuring a channel necessary for the terminal to measure a downlink CSI, and was introduced in the 3GPP LTE Release 10. The CSI-RS is also referred to as an NZP CSI-RS in order to distinguish the CSI-RS and a zero-power (ZP) CSI-RS that will be described later. Although a 3GPP LTE Release 8/9 system has used a cell-specific reference signal (CRS) in order for the terminal to measure the CSI, an introduction of a new reference signal for estimating a channel having a lower density than the existing CRS in order to support downlink transmission of a maximum of 8 layers has been required from the 3GPP LTE Release 10.
The terminal is configured with the CSI-RS through user equipment (UE)-specific radio resource control (RRC) signaling. Numbers of CSI-RS antenna ports with which the terminal may be configured are 1, 2, 4, and 8. The CSI-RS is transmitted in the entire region of a system bandwidth. Two resource elements (REs) are used per physical resource block (PRB) pair for the CSI-RS transmission of each of the CSI-RS antenna ports. Code division multiplexing (CDM) is performed on 2 CSI-RS antenna ports across 2 REs that are located in two consecutive orthogonal frequency division multiplexing (OFDM) symbols (or that are apart from one OFDM symbol) on the same subcarrier, and thus the 2 CSI-RS antenna ports have density of 1 RE/CSI-RS antenna port. A transmission period in a time axis of the CSI-RS may be configured as 5, 10, 20, 40, or 80 ms. Mapping of the REs on which the CSI-RS is transmitted follows a pattern defined by a configuration parameter (CSI-RS configuration) for each antenna port.
An interference measurement (CSI-IM) resource are a resource for measuring interference necessary for the terminal to measure the CSI, and was introduced in the 3GPP LTE Release 11. Positions of REs in the PRB pair for CSI-IM are indicated through a CSI-RS configuration parameter corresponding to 4 CSI-RS antenna ports. A transmission period in a time axis of CSI-IM may be configured as 5, 10, 20, 40, or 80 ms, like the NZP CSI-RS.
It is assumed that in a case where the terminal performs physical downlink shared channel (PDSCH) rate matching, the PDSCH is not mapped to an RE configured as a ZP CSI-RS. The ZP CSI-RS may be largely used for the following two purposes. First, the base station does not transmit (or performs muting on) a signal in an RE on which the CSI-RSs of adjacent cells are transmitted, thereby improving CSI-RS measurement performance of the terminal in the adjacent cells. Hereat, the base station may inform the terminal of REs to which muting is applied through the ZP CSI-RS configuration. Second, ZP CSI-RS may be configured for the purpose of configuring resources for interference signal measurement of the terminal. The terminal may be configured with CSI-IM resources used to measure an interference signal only in a ZP CSI-RS resource region.